A creature has Evasion:
‘If she makes a successful Reflex saving throw against an attack that normally deals half damage on a successful save, she instead takes no damage.’
And also has Vulnerability to Energy:
‘Some creatures have vulnerability to a certain kind of energy effect (typically either cold or fire). Such a creature takes half again as much (+50%) damage as normal from the effect, regardless of whether a saving throw is allowed, or if the save is a success or failure.’
Which rule takes precedence if a creature is attacked by an effect of that energy type and a successful Reflex saving throw is made against this energy attack that normally deals half damage on a successful save?